As with internal combustion engines, the individual components of automatic transmissions are manufactured in such a way that they intermesh in an ideal way within a particular temperature range. Since the temperature of the transmission increases in operation and with increasing load and revolutions, the transmission oil situated therein must be cooled. For this purpose, the transmission must be fluidly connected to a suitable air/oil cooler, for example. Owing to the increasing dimensions of modern cooling systems in motor vehicles, these are sometimes sufficient to be used for cooling the transmission as well. Suitable devices for this purpose are heat exchangers designed as water/oil heat exchangers, which are used for heat transmission between the coolant circuit of the internal combustion engine and the transmission oil.
In this context, there are efforts being made to use such a construction to heat the transmission oil in the cold starting phase as well. The goal is to reach the operating temperature quickly so as to reduce the resistance within the transmission and hence to reduce energy consumption. Since the coolant in the cooling circuit of the internal combustion engine heats up more quickly than the transmission oil, the heat exchanger provides heat transfer from the coolant to the transmission oil. The heat exchanger thus not only performs the task of cooling transmission oil but, more generally, temperature control thereof. In respect of the arrangement of a heat exchanger of this kind and the operation thereof, a number of embodiments are already known in the prior art.
Thus, according to DE 103 01 448 A1, temperature control of the transmission oil is achieved by providing at least two thermostats. Here, one of the thermostats is associated with a heat exchanger, which serves to exchange heat between the transmission oil and a coolant. Particularly in the cold starting phase of the associated internal combustion engine, the other thermostat serves to allow the coolant to circulate in relation to the internal combustion engine only within a small cooling circuit. Thus, it is only when a predetermined temperature of the coolant is reached that the thermostat is opened and a supply of heated coolant to the heat exchanger is allowed.
JP 2004 232 514 A furthermore discloses an arrangement for controlling the temperature of transmission oil for an automatic transmission of a motor vehicle. The aim is rapid heating of the transmission oil after the internal combustion engine is started so as to reduce fuel consumption when cold starting. For this purpose, a heat exchanger is provided, allowing heat exchange between the cooling water of the internal combustion engine and transmission of the automatic transmission. The arrangement furthermore requires a switching valve with a control system, wherein the switching valve is arranged on a cooling water inlet of the heat exchanger. The switching valve is designed to switch the flow of the cooling water into the heat exchanger between a first and a second cooling water circuit. In this case, the first cooling water circuit corresponds to the cylinder head and the second cooling water circuit corresponds to the engine block of the internal combustion engine. The switching valve is controlled in such a way that the heat exchanger is fluidly connected to the first cooling water circuit to heat the transmission oil and to the second cooling water circuit to cool the transmission oil.
A temperature control arrangement can also be found in US 2011/0120396 A1, said control arrangement having a heat exchanger unit fluidly connected to a coolant circuit. The heat exchanger unit has a heat exchanger part and a valve body coupled to the heat exchanger part. Arranged within the heat exchanger part is a heat exchanger. The heat exchanger is designed to allow heat exchange between transmission oil and a coolant circulating within the coolant circuit. The valve body, in particular, is designed to transfer the coolant leaving an associated internal combustion engine to the respectively appropriate location. For this purpose, an orifice is arranged within the valve body, being used to control the flow of the coolant as a function of the temperature of the coolant and transmission oil. A bypass which bypasses the heat exchanger part is furthermore provided, allowing the coolant to flow out of the internal combustion engine, via a coolant pump, and back into the internal combustion engine without releasing heat to said heat exchanger, particularly in the cold starting phase.
The arrangement of the heat exchanger according to the teachings indicated above allows the transmission oil to be heated after the internal combustion engine has reached its operating temperature. Thus, common to them all is the wait for a predetermined temperature of the coolant to be achieved before said coolant is used to supply the heat exchanger.
Further-reaching teachings envisage heating up the internal combustion engine and transmission in parallel, as described in U.S. Pat. No. 5,638,774 A, for example. This discloses a temperature control arrangement for transmission oil in an automatic motor vehicle. For this purpose, an internal combustion engine and a cooler are provided, which are fluidly connected to one another by a coolant circuit. The coolant circuit contains a thermostat arranged downstream of the cooler. The thermostat is designed to direct a coolant circulating in the coolant circuit at least partially through the cooler and/or, while bypassing the cooler, at least partially through a bypass and back to the internal combustion engine. In order to achieve heat exchange between the coolant and the transmission oil, a corresponding heat exchanger is furthermore provided. Said heat exchanger is arranged within a bypass which bypasses the internal combustion engine. This extends from a segment extending between a heat pump and the internal combustion engine to a coolant circuit segment situated downstream of the internal combustion engine. As a result, the bypass is arranged between the feed section and the return section of the coolant circuit, it being possible for the coolant passing through the cooler and bypassing the cooler to be directed at least partially through the heat exchanger before entering the internal combustion engine. The coolant leaving the heat exchanger is reintroduced into the coolant circuit after the internal combustion engine.
US 2008/0276886 A1 has also disclosed a cooling system for an internal combustion engine, which provides simultaneous heating of transmission oil. In this case, a coolant circuit is used to connect the internal combustion engine fluidly to a cooler. A coolant circulating within the coolant circuit is directed either through the cooler or past the latter with the aid of a thermostat, depending on the temperature of the coolant. Also provided is a heat exchanger, which allows heat exchange between the coolant and the transmission oil in an automatic transmission coupled to the internal combustion engine. For this purpose, the heat exchanger is arranged within a bypass, which extends between the coolant feed toward the cooler and the return from the cooler. Here, the bypass is embodied in such a way that the flow through said bypass, together with the heat exchanger, is in opposite directions, depending on the switching state of the thermostat. As a result, the coolant leaving the internal combustion engine can be directed back to the internal combustion engine, either bypassing the cooler in the case where the thermostat is closed or flowing through the cooler when the thermostat is open and then flowing through the heat exchanger.
By means of the two last-mentioned arrangements of the heat exchanger within the coolant circuit, coupling of the individual phases of temperature control of the coolant and the transmission oil is achieved. For this purpose, the arrangement of a bypass is proposed, extending in each case between the feed and the return of the coolant to the cooler and away from the latter. Owing to the large number of restrictors required for the intended routing of the coolant, US 2008/0276886 A1 demands a relatively complex construction. In addition to this, there are the correspondingly high costs of materials and the increased weight of such an embodiment. In addition, there is the required reversal of direction of the coolant, which requires precise construction of the resistances in the conduits to obtain adequate cooling performance. From the point of view of quick heating of the transmission oil when cold starting, U.S. Pat. No. 5,638,774 A has long loss paths before coolant leaving the internal combustion engine reaches the heat exchanger.
Thus, the heating and cooling performance of the coolant in respect of quick attainment and subsequent holding of the operating temperature of the internal combustion engine, in particular, are affected by the respective construction of such an arrangement. Moreover, such a construction should be embodied in a way which is as simple as possible and yet efficient. Accordingly, the arrangement of a heat exchanger for heat transfer between the coolant and the transmission oil and the operation of a temperature control arrangement of this kind still leave room for improvement.
Given this background, it is the underlying object of the invention to improve a temperature control arrangement for transmission oil in a motor vehicle and a method for controlling the temperature of transmission oil in a motor vehicle in such a way that efficient temperature control of the transmission oil in respect of the rapid heating thereof and in respect of efficient cooling is made possible, despite a simple construction of the arrangement and without additional components.